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Five Syrian men detained in Honduras for traveling on doctored Greek passports do not belong to "any terrorist cell," Honduran authorities said on Thursday, adding that four were university students.

They said the five had been fleeing conflict in Syria and had been trying to get to the United States when they were stopped and detained.

Honduran police said there were no signs of any links to last week's suicide bombings and shootings in Paris that killed 129 people. IS claimed responsibility for the attacks.

The five were part of a wider group of seven Syrian nationals who acquired forged passports in Brazil and then went by land to Argentina on their way north, a US government source familiar with the case said. The source also said there was no evidence to suggest the men were militants.

They took advantage of the porous Triple Frontier near well-traveled tourist destination Iguazu Falls where Brazil, Argentina and Paraguay meet, the source said. The three countries belong to the Mercosur customs bloc, where it is possible to cross borders with simple identity documents rather than passports.

"They probably spent the majority of their time in Brazil, because that's where they got the documents. They made their way from Brazil through Argentina pretty quickly," the US government source told Reuters. "There's a human smuggling cell that is moving people through the region."

Five of the men were detained late on Tuesday in the Honduran capital, Tegucigalpa, on arrival from Costa Rica, police said. They said passports had been doctored to replace the photographs with those of the Syrians.

They arrived in Brazil from Turkey about a week ago, the US government source said.

A Honduran official said the five men detained there had spent several days in Costa Rica before flying into Honduras.

A sixth man was turned away from Honduras on Friday on arrival by plane from El Salvador, and was sent back. In Paraguay, police detained a seventh Syrian man who was traveling on a stolen Greek passport.

The source said that the Syrian detained in Paraguay was caught because his fake passport was missing some of the vaccinations required.

"They made it through and these are the ones we caught. We got lucky on these guys. Think about the ones we didn't' catch. It's a huge concern," the source said.

Separately, members of two Syrian families turned themselves in to US authorities in Laredo, Texas, at the Mexican border, US Homeland Security officials said on Thursday.

The Department of Homeland Security said two men, two women and four children "presented themselves" at the port of entry and were taken into custody by Border Patrol agents before being handed over to immigration officials for processing.